ycbm
Overwhelmed
Get your vet to do a full blood panel. If you have a free call out day even better. For £60 you can work from a factual basis rather than guess work.
Where do you get a full mineral panel from for that money? I pay £10 per mineral after paying for the blood sample.
Also, my vets tell me that there are no accepted figures for minerals in horses blood, so they use cow ones. And I can confirm what Ester says, that horses can look just fine and dandy and be high in iron, manganese and selenium and low in copper.
I wouldn't trust a feed company to tell me how to feed my horses further than I could pick the rep up and throw them.